Radio frequency filters are components frequently used in communications devices, and have many types and forms. Metal coaxial cavity filters in the radio frequency filters are applied to radio frequency front-ends of high-power wireless communications base stations due to their desirable performance indicators (including an insertion loss and a power capacity).
As wireless communications technologies develop, wireless communications base stations are distributed in an increasingly dense manner. It is required that volumes of the base stations become increasingly small. A radio frequency front-end filter module occupies a relatively large proportion of a volume of a base station. Therefore, a filter is also required to have an increasingly small volume. However, when a volume of a metal coaxial cavity filter is reduced, it is found that a smaller volume of the filter results in a greater surface current, a greater loss, and a lower power bearing capability, that is, a smaller power capacity. That is, with a decrease in the volume of the metal coaxial cavity filter, performance indicators of the metal coaxial cavity filter deteriorate.
At present, there is a miniaturized filter that uses a body made of a solid dielectric material and a resonator that is formed by metalizing (for example, by plating silver) a surface of the body (a solid dielectric resonator). Multiple resonators and coupling between the resonators form a filter (e.g. a solid dielectric filter). The coupling between the resonators may be classified by polarity, for example as positive coupling (which may also be referred to as “inductive coupling”) and negative coupling (which may also be referred to as “capacitive coupling”). A transmission zero may be formed based on a polarity of coupling between the resonators. The transmission zero refers to a frequency outside a passband of a filter, and a degree of suppression that is applied by the filter on a signal at the frequency is theoretically infinite. The addition of a transmission zero can effectively enhance a near-end suppression capability of the filter (that is, a suppression capability of a frequency near the passband). For example, in a three-cavity filter with resonators 1, 2, and 3, if coupling between resonators 1 and 2, coupling between resonators 2 and 3, and coupling between resonators 1 and 3 are positive coupling, a transmission zero is formed on the upper side of a passband. However, if the coupling between the resonators 1 and 2, and the coupling between the resonators 2 and 3 are positive coupling, and the coupling between the resonators 1 and 3 is negative coupling, a transmission zero is on the lower side of the passband. To implement negative coupling, structures shown in FIG. 1a and FIG. 1b are currently used in a solid dielectric filter. A mechanical part 10 (FIG. 1a) having at least one surface of which is metalized, is connected between two solid dielectric resonators 11 and 12 (FIG. 1a). The two solid dielectric resonators are separated by using a groove 13. The resonator 11 and the mechanical part 10 are coupled by an electric field, to form a current on the mechanical part 10. The current flows to the resonator 12 along the mechanical part 10. The mechanical part 10 and the resonator 12 are coupled by an electric field, thereby forming capacitive coupling between the two resonators.
However, because the interior of the solid dielectric resonator is a solid medium instead of air, and the solid medium is formed by die casting, an implementation technique of a metalized mechanical part inside the solid medium is very difficult, and a coupling degree of the capacitive coupling cannot be adjusted.